The particular structure of the interfacial shear as a closure law in two-fluid models ought to bridge the gap between microscale phenomena and the macro-averaged representation of the flow. Conventionally, quasi-steady models of the interfacial shear are adopted in relation to the local phase holdup and velocities. The main attempts to account for the complicated interfacial interactions have been focused on improving the friction factor modelling. The present study suggests incorporation of an explicit functional dependence of the interfacial shear on the interface slope due to interfacial waviness. The implementation of the proposed model as a closure law in the stability analysis of stratified flows reveals the crucial role of the dynamic term in determining the stability characteristics. It is shown that with the inclusion of the newly proposed dynamic term of the interfacial shear the stratified smooth-stratified wavy transitional boundary is satisfactorily predicted for a wide range of two-fluid horizontal and inclined systems.